This section is intended to introduce various aspects of the art, which may be associated with some embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
There was never a shortage of challenges facing the offshore industry in deep-water or arctic frontiers. Nowadays, however, the challenge is particularly daunting with the merger of the two frontiers in new arctic deep-water leases, such as the Beaufort Sea, Chuckchi Sea, Kara Sea and elsewhere. The main challenge can be succinctly summarized in two words: station-keeping.
Take, for example, a concrete island drilling structure. This shallow-water mobile offshore drilling unit was designed by Global Marine to withstand about 59,000 tonnes of ice load in around 17 meters of water depth. Wetmore, 1984. As water depth increases, ice loads do not increase much, but overturning moment does typically prohibit the use of fixed-base structures beyond a limiting depth of about 100 m. In fact, the 100 m depth typically delineates the boundary between shallow and deep zones in arctic waters. As a result, this shifts the focus for the developers of those leases to floating platforms and drill ships.
The next boundary that faces the industry is that of mooring system capacity. Floating systems, such as Hoover-Diana, are held in place by a mooring system with a capacity in the range of 1,000-2,000 tonnes. See API RP2SK, 2005. Unfortunately, this capacity pales in comparison with sea ice load magnitudes, not to mention iceberg loads. Dynamically-positioned drill ships rely on azimuthing thrusters for station-keeping. While the azimuthing thrusters are capable of adequate station-keeping on non-ice conditions, they have much lower capacity to withstand ice loads.
Subsea developments may end up being the viable concept for arctic deep-water development, but some operations still need to be conducted at the surface: i.e., a drillship drilling a subsea well, a tanker loading crude, or other. As a result, offshore operations utilizing known technology have to consider operating only during the open water season.
In higher arctic, however, the season exhibits a scarcity and a variance of open water weeks over the years. At the Ajurak block in the Canadian Beaufort Sea, for example, the open water season lasts between 0 and 24 weeks, with a median of about 9 weeks. Further, the open water season may simply not be sufficient; other means, such as ice management, will likely have to play an essential role to extend the operational season beyond the period of open water, as well as to protect operations against ice floe intrusions during the open water season.
Thus, there is a need for improvement in this field.